Consistently increasing costs of petroleum and natural gas and concern about the continued availability of those energy sources and dwindling reserves of them have led many larger scale consumers of energy to explore alternate sources of energy such as oil shales and tar sands; solar, wind, wave, tidal, and goethermal energy; and nuclear energy (especially breeder reactors). Also, much attention has been focussed on the increased use of coal, once this world's dominant energy source, because of the huge reserves of that fuel. Also, while coal has a number of drawbacks as an energy source--environmental degradation and atmospheric pollution, health, safety, transportation, storage and waste (ash) disposal problems--so do the alternative energy sources identified above, and they are consequently not widely considered as viable alternatives to coal at the present time in most energy consuming applications. Research into methods of utilizing coal that will minimize its negative characteristics have consequently been continued and perhaps even intensified.
One approach to the utilization of coal as an energy source that is currently receiving attention is fluidized bed combustion. In that technique the bottom of a combustor is filled with a bed of a granular, inert, particulate material (sand, limestone, and ash are commonly used); and air is blown upwardly through the bed, converting it into one that to some extent has properties akin to those of a fluid. Coal (or other fuels) can be fed into, and burned in, a fluidized bed, liberating thermal energy that can be used for any of a variety of purposes.
Additives can be used to control unwanted emissions from a fluidized bed combustor. For example, if crushed limestone or dolomite is fed to the fluidized bed with a sulfur-containing coal, the sulfur will react with the calcium in the additive to form calcium sulfate. This is a solid and can be removed from the combustor with the ash generated in the combustion process.
In more recent years the fluidized bed has on occasion been housed in a pressurized shell, making the combustor capable of generating gases which can be used to drive the turbine(s) of a gas turbine engine. Consequently, the use of a pressurized fluid bed reactor to generate the working fluid for a gas turbine engine offers an opportunity for lessened dependence on such sources of energy as imported oil and natural gas.
Pressurized fluid bed combustors are efficient and can be made capable of producing gases which are low in NO.sub.x and sulfur content; e.g., by employing appropriate additives as described above. However, contamination of the product gases with other substances which are capable of causing turbine degradation by erosion, fouling, and hot corrosion remains a problem in gas turbine engine applications. Both hard and soft (and molten) particles in the gases can cause erosion of turbine components while soft particles of materials such as calcium sulfate cause fouling, and other substances such as sodium sulfate cause hot corrosion. As a result, turbines operated on gases generated by the combustion of solid fuels have heretofore not had an acceptable service life.